I bought a new 2015 Chevy Spark 1.2 liter manual transmission. It is now 5 years old and has 60,000 miles on it. It has a rough idle. I got the car back from service at a delership and the air cleaner bolts were loose and the throttle body had a ring of sand. So, they ran the engine with the air cleaner off. I cleaned it and still had a rough idle. So, I tried 91 octane gas and it helped the idle some and I get better aceleration. I found that my AC Delco battery had 263 amps (no engine light). I replaced it. I still have a rough idle. Now I am looking at any data I can get with my scanner. There are no trouble codes and the engine light is off. I did a google searchon the EVAP_VP(pa) readings and could not find much infrmation online. How do the following EVAP_VP(pa) readings look? And if the EVAP_VP(pa) were to go down to -265 should I have a concern of a high or rough idle. In other words is EVAP_VP a measure of vacuum (pulling gas fumes into the engine) and is this range normal -10 to -120)? I revved the engine up and let the gas pedal go back down. There is a clapping nosie when the engine deccelerates. Here are some frames on a cool day and 3/4 tank of gas with (RPM, EVAP_PT(%), EVAP_VP(pa) ). I notice in the data the EVAP_PCT goes up with idle and the EVAP_VP goes more negative. If the EVAP_PT(%) were to go to zero and the EVAP_VP(pa) vacuum goes way low to -260 would that indicate a problem?
(799,19,-41)
(965,24,-57)
(2031,24,-63)
(2705,31,-68)
(2727,37,-99)
(2643,38,-93)
(2529,33,-99)
(1465,22,-83)
(1414,23,-69)
(1008,21,-53)
(917,20,-62)
(829,19,-59)
EVAP_VP(pa)
Short for EVAP system vapour pressure, measured in Pascals (Pa) which is a metric unit of pressure.
EVAP is connected to engine vacuum so it shouldn't stay at zero. Vacuum is greatest at idle and coasting when the throttle is shut. On acceleration, throttle opens and manifold pressure approaches atmospheric pressure. So from your description, it sounds normal.
Rough idle can be a lot of things, so pick off the low hanging fruit first. Check the fuel pressure, check spark plugs and ignition system.
On my car 2009 HHR the EVAP_VP measures pressure in the gas tank or some tank near the gas tank, like a charcoal filter or something. My question is if the evap percentage which is the PCM signalling to open or close the purge solenoid goes to 0% and the vacuum on evap_VP remains very negative like -265 should I think that the purge solenoid is not responding to the signal from the PCM.
maybe you can pull it out and bench test it.